Use flalign or alignat or align or similar environment to align to the left

You can use Flalign instead.

The formula is
\begin{flalign*}
a &= b+c &\\
  &= 1+1 &\\
  &= 2  &
\end{flalign*}

Using the flalign environment (numbered equations) or the flalign* (unnumbered equations) environment, left alignment can be obtained with two ampersands (&). If the only alignment you want is for the equations to be left-aligned, put an ampersand at the beginning and end of each line, like so:

\begin{flalign*}
  & \lambda = \lambda_1 + \lambda_2 &\\
  & \Lambda = \Lambda_1 + \Lambda_2 &
\end{flalign*}

If you want to have left-alignment but also align by some character (say, the = sign), put the first ampersand immediately before the character you want to align by, and the second ampersand at the end of each line, like so:

\begin{flalign*}
  \lambda &= \lambda_1 + \lambda_2 &\\
          &= \Lambda &\\
          &= \Lambda_1 + \Lambda_2 &
\end{flalign*}

---

Credit to Lauer, whose answer I used to help me figure out the same issue, then expanded upon.

The trick is to give amsmath the parameter fleqn:

\usepackage[fleqn]{amsmath}

Edit: This produces a slightly different output than flalign*.